1. Field of the Invention
The invention relates generally to a positioning apparatus and a positioning method for use in photolithography, among other applications. More particularly, the invention is directed to a positioning apparatus for a wafer table, the positioning apparatus including an electromagnetic bearing system which effectively provides fine position control of the wafer table.
2. Description of Related Art
Many applications, particularly in semiconductor manufacturing, require precision positioning of an object. They include scanning tunneling microscopy, optical inspection, and photolithography. During photolithography, a wafer table is commonly used to position a wafer precisely with respect to the photolithographic apparatus.
The positioning and alignment of the wafer table can be performed in various ways. In U.S. Pat. No. 5,294,854, the entire contents of which are incorporated by reference herein, an electromagnetic bearing system is used for fine position control of the wafer table. The electromagnetic bearing system used in this patent allows for positioning and movement control of the wafer table in multiple degrees of freedom. Specifically, the wafer table is moved into desired positions by applying different currents to electromagnets that interact with corresponding adjacent magnetic members that are attached to the wafer table.
The magnetic members disclosed in the '854 patent have a rectangular shape (see FIG. 4). When the wafer table is moved horizontally (e.g., in the X- or Y-direction) and vertically (in the Z-direction), the wafer table sometimes becomes tilted. When the wafer table becomes tilted, the magnetic member becomes tilted with respect to the electromagnets, and the gap distance between the magnetic member and the electromagnets differs along the height of the magnetic member. The difference in the gap distances in turn causes the magnetic coupling across the magnetic member to change and a torque develops along the magnetic member. The resulting torque can affect the precision positioning of the wafer table, and so, to compensate for this torque, additional electrical current is applied to the electromagnets.
While the prior art attempts to solve the torque problem by adjusting the electrical current in the electromagnets, this can result in unnecessary wafer table flutter, an increase in the chance of inducing distortion to the wafer table, and undesirable heat in the electromagnet bearing and the attached members. Therefore, it is desirable to have an electromagnetic bearing system that can minimize the torque created along the magnetic member.